1. Field of the Invention
The present invention relates to compressed gas cylinders. In particular the present invention relates to a compressed gas cylinder including a cap having a domed area configured to reduce the force required to release the compressed gas stored within the cylinder.
2. Background of the Invention
Small compressed gas cylinders, or microcylinders, are well known in the art. Because they are capable of storing a considerable volume of a chosen gas at high pressure, microcylinders provide a compact but powerful energy source, and, as a result, microcylinders are presently used in a wide range of applications. For example, microcylinders are presently used as the energy source in emergency inflation devices, gas powered rifles and handguns, tire inflation devices, pneumatically driven injection devices, and even in devices for whipping cream. Despite being pressed into service in several different functional contexts, however, state of the art microcylinders are not ideally suited to each of the applications in which they are presently used.
In particular, state of the art microcylinders are not ideally suited for use in automatic injection devices, otherwise known as “autoinjectors.” Autoinjectors are generally designed to facilitate quick, automatic, and accurate injection of a desired dose of a chosen medicament and are thought to be particularly well suited for use in emergency situations or by subjects who must regularly self-administer therapeutic substances. Where the design of the autoinjector or the nature of the medicament to be delivered by requires either that the autoinjector accelerate the medicament to a high velocity or that the autoinjector drive the medicament with a high injection force, microcylinders are thought to be ideal candidates as energy sources for the autoinjector. However, in order to release the compressed gas from within a microcylinder, the microcylinder must be pierced, or otherwise compromised, and the caps or seals typically included on microcylinders cannot be pierced without the application of relatively high force.
A standard microcylinder is illustrated in FIG. 1 through FIG. 3. The microcylinder illustrated in these figures is exemplary of microcylinders available through several commercial suppliers, such as Leland Limited, Inc., of South Plainfield, N.J. As can be seen in FIG. 1 through FIG. 3, the standard microcylinder 10 includes a body 12 terminating in a cap 14. In order to release the compressed gas stored within the microcylinder 10, the cap 14 is generally pierced, and to reduce the force necessary to pierce, the cap 14, the cap 14 may be-provided with an area of reduced thickness, or “pierce region” 16, where the cap may be pierced more easily (shown in cross-section in both FIG. 2 and FIG. 3). Because the pressure exerted by the gas stored in a standard microcylinder 10 places the material forming the pierce region 16 in tension (indicated by arrows 17), however, the extent to which the pierce region 16 can be thinned is limited, as the pierce region 16 must be sufficiently strong to resist tearing when exposed to the tensile forces exerted by the gas compressed within the microcylinder 10. Therefore, even where the cap 14 of a standard microcylinder 10 is provided with a pierce region 16, the force required to pierce the cap 14 can exceed fifteen pounds, or more.
To overcome the pierce force problem created by standard microcylinders, autoinjectors including standard microcylinders generally include a mechanism that facilitates the generation of a force sufficient to pierce the microcylinder cap. The mechanism itself may be designed to generate a force sufficient to pierce the microcylinder, as is exemplified in the autoinjector taught in U.S. Pat. No. 6,096,002, or the mechanism may simply impart a mechanical advantage sufficient to enable the user to exert the required pierce force through the exertion of a smaller force. Such mechanisms, however, are generally not desirable, as they may complicate the design of the autoinjector and may, in some cases, prove to be an inconvenience to the user.
In an attempt to cure the problems presented by the high pierce forces required by standard microcylinders, The BOC Group of Windlesham, United Kingdom, developed microcylinders including a frangible, or breakaway, cap. U.S. Pat. No. 5,845,811 (“the '811 Patent”) and U.S. Pat. No. 6,047,865 (“the '865 Patent”) are directed to two different breakaway microcylinders 18, 20 developed by the BOC Group, the two different designs being illustrated herein in FIG. 4 and FIG. 5. The first design 18, which is described in the '811 patent, includes a cylinder body 12, a cap 14 including a frangible area 22, a lever 24, and an anchor member 26. The cap 14 is compromised through application of a force to the lever 24, which causes the frangible area 22 to fracture. The second design 20, which is described in the '865 Patent, includes a microcylinder having a body 12, a cap 14 with a frangible area 22, and an elongated neck 28. The elongated neck 28 of the second design effectively replaces the lever 24 of the first design, with the frangible area 22 being fractured as a force is applied to the elongated neck 28. Relative to a standard microcylinder, the designs proposed in the '811 and '865 Patents reduce the amount of force that a user must apply to compromise the microcylinder.
However, like standard microcylinders, the breakaway microcylinders taught in the '811 and '865 patents are not without disadvantages. In particular, both the lever of the first design and the elongated neck of the second design are exposed, which increases the risk that the break-away microcylinders will be accidentally compromised, or “fired,” as they are handled, for example, during transportor during a device assembly process. It would, therefore, be an improvement in the art to provide a gas cylinder that is not only capable of storing a compressed fluid or gas at high pressures, but which also includes a cap that is relatively difficult to accidentally fire and can be pierced through the application of a relatively small force.